Frame foot loading measurement system using fiber optic sensing technique

ABSTRACT

A fiber Bragg grating (FBG) based sensor is used as a strain sensing element to determine frame foot loading of a generator. Three FBGs may be used in tandem to form a basic Frame Foot Loading Module (FFL Module). Two modules are fixed on each vertical support gusset at the corner of the generator frame, with one module on the front of the gusset and a second module on the back of the gusset. Thus, each gusset may be instrumented with six FBG strain gauges or sensors. The gussets are chosen on each of the four corners of the generator. For two-pole generators the first three gussets at each corner may be used and, for four-pole generators the first four gussets may be used.

FIELD OF THE INVENTION

The present invention relates to the field of generators and, moreparticularly, to monitoring frame foot loading of turbine poweredgenerators.

BACKGROUND OF THE INVENTION

A turbine powered generator, or turbo-generator, for energy conversionincludes a frame structure that is typically installed over a concretefoundation that provides the needed structural support for thegenerator. There are usually one or more feet along portions of theframe that help transfer the load of the generator frame to thefoundation. The weight supported by the generator feet is typicallytransmitted to the foundation through a shim pack, seating plate andgrouting. Varying the shim pack thickness permits alignment between thegenerator and the turbine during erection and maintenance. The seatingplate, grouted to the foundation during erection, provides a solid baseof support for the generator. Frame feet that may extend the full lengthof the frame are typically loaded uniformly, using shim packs whoseuniform thickness is modified only to obtain final alignment of thegenerator to the turbine. Thus, the stator core weight and electricalload are carried by the central portion of the generator while the frameends support the rotor in the bearings.

To minimize the generator shaft bearing span and to increase stiffness,the rotor bearings can be supported by respective brackets on each endof the frame structure, instead of being supported by external bearingpedestals. This arrangement of the bearings means that the frame feet atthe ends of the generator should provide solid support for the rotorshaft and bearings. In the past, electro-mechanical strain gauges havebeen used on frame ribs, or gussets, to measure load distribution oneach foot and to optimize the foot's position for dynamic bearing loads.A load distribution pattern based on frame deflection is used for properframe foot loading. In particular, one or more electro-mechanical straingauges have been used on one or more gussets that are located near thecorners of the frame structure; it is these vertical support gussetsthat bear the frame weight at the corners.

The use of electro-mechanical strain gauges in the manner describedabove introduces some reliability and operational constraints. First,the standard electro-mechanical strain gauges are typically bonded tothe gusset substrate via a hydroscopic cement, that can sometimes fail.Even when care is taken to coat the strain gauges with a sealant to keepmoisture out, the bond life of the cement can be as brief as 12-18months. Thus, generators that have been frame foot loaded in the pastwill need to have the old strain gauges removed and new strain gaugesinstalled for future frame foot loading.

Also, the installation of standard electro-mechanical strain gauges istime consuming even for an experienced technician. A standard straingauge installation by experienced field personnel is estimated to beabout 1 hour for each strain gauge. Thus, by way of example, a typicalinstallation for a four-pole generator may include up to 64 straingauges to properly instrument the gussets and can therefore requiresignificant installation time.

Also, by design, each standard strain gauge typically requires 3 wiresfor measurement. For a four-pole generator, as many as 256 wires mayneed to be routed from the generator to the strain gauge analogconnectors. These connections also introduce a large amount of timeneeded for making final measurements.

Thus, there remains the need to perform frame foot loading measurementsfor a power generator in a fast, efficient and accurate manner and in away that ensures reliable results for long periods of time.

SUMMARY OF THE INVENTION

Aspects of the present invention relate to a strain measuring modulethat is based on fiber Bragg grating sensors. The module includes anoptical signal path having a first end and a second end with a firstfiber Bragg grating in the optical signal path between the first end andthe second end, and a second fiber Bragg grating in the optical signalpath between the first fiber Bragg grating and the second end. Themodule also includes a housing configured to substantially enclose thefirst and second fiber Bragg gratings; wherein the housing has an outersurface configured to be mechanically attached to a surface of a supportgusset of a generator. Further, a temperature sensor may be providedlocated proximate to the first and second fiber Bragg gratings.

In accordance with an additional aspect of the invention, a strainmeasuring device is provided comprising a plurality of modules. Each ofthe modules comprises an optical signal path having a first end and asecond end, and at least one fiber Bragg grating in the optical signalpath between the first end and the second end. Each module additionallyincludes a housing configured to substantially enclose the at least onefiber Bragg grating; wherein the housing has an outer surface configuredto be mechanically attached to a surface of a support gusset of agenerator. The plurality of modules are arranged in a sequential chainof adjacent modules having a beginning module and an ending module toprovide a single optical signal path. A light source is coupled with thefirst end of the beginning module and is configured to provide anincoming spectrum of light. A detector is coupled with the first end ofthe beginning module and is configured to receive a respective reflectedsignal from each of the plurality of modules corresponding to therespective Bragg gratings. A jumper fiber is configured to opticallycouple the second end of each of the plurality of modules to the firstend of its respective neighbor in the sequential chain, starting at thebeginning module and stopping at the ending module.

Yet another aspect of the present invention relates to a method ofdetermining frame foot loading of a generator including a generatorcasing supported on a plurality of frame feet and including gussetsextending between the generator casing and the frame feet. The methodincludes attaching at least one frame foot loading module to each of aplurality of the gussets, the plurality of modules being arranged in asequential chain of adjacent modules having a beginning module and anending module to provide a single optical signal path. Each of themodules comprises an optical signal path having an individual first endand a second end; at least one fiber Bragg grating in the optical signalpath between the first end and the second end; and a housing configuredto substantially enclose the at least one fiber Bragg grating; whereinthe housing has an outer surface configured to be mechanically attachedto a surface of a support gusset of a generator. The method additionallycomprises coupling a light source with the first end of the beginningmodule, the light source being configured to provide an incomingspectrum of light; coupling a detector with the first end of thebeginning module, the detector being configured to receive a respectivereflected signal from each of the plurality of modules corresponding thea respective Bragg grating; and attaching a jumper fiber to opticallycouple the second end of each of the plurality of modules to the firstend of its respective neighbor in the sequential chain, starting at thebeginning module and stopping at the ending module.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the present invention, it is believed that thepresent invention will be better understood from the followingdescription in conjunction with the accompanying Drawing Figures, inwhich like reference numerals identify like elements, and wherein:

FIG. 1 is a conceptual view of a fiber Bragg grating that can be used asa strain gauge in accordance with the principles of the presentinvention;

FIG. 2 is a conceptual view of a sensor arrangement of a plurality offiber Bragg gratings that can be used as a strain gauge arrangement inaccordance with the principles of the present invention;

FIG. 3 depicts a weldable fiber Bragg grating component in accordancewith the principles of the present invention;

FIGS. 4A-C depict a plurality of fiber Bragg gratings arranged in amodule in accordance with the principles of the present invention;

FIGS. 5A and 5B are each a perspective view of different areas of aturbo generator;

FIG. 6 depicts two gussets having frame foot loading modules inaccordance with the principles of the present invention;

FIG. 7 depicts a corner of a generator frame with a plurality of gussetsand frame foot loading modules in accordance with the principles of thepresent invention; and

FIG. 8 is a flowchart of an exemplary method of sensing frame footloading patterns in accordance with the principles of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiment,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration, and not by way oflimitation, a specific preferred embodiment in which the invention maybe practiced. It is to be understood that other embodiments may beutilized and that changes may be made without departing from the spiritand scope of the present invention.

In accordance with aspects of the present invention a fiber Bragggrating (FBG) based sensor is used as a strain sensing element todetermine frame foot loading of a generator, such as a turbo-generator.Three FBGs may be used in tandem to form a basic Frame Foot LoadingModule (FFL Module). However, it should be understood that the FFLModule may comprise a single FBG. Two modules may be fixed on eachvertical support gusset at the corner of the generator frame, with onemodule on the front of the gusset and a second module on the back of thegusset. Thus, each gusset may be instrumented with six FBG sensors. Thegussets are chosen on each of the four corners of the generator. Fortwo-pole generators, the first three gussets at each corner may be usedand, for four-pole generators, the first four gussets may be used.

FIG. 1 is a conceptual view of a fiber Bragg grating that can be used asa strain gauge or sensor in accordance with the principles of thepresent invention. A fiber Bragg grating (FBG) 104 is typically formedin the core 106 of an optical fiber 102. The core 106 is surrounded bycladding 108 as shown in FIG. 1. These gratings are passive opticaldevices that can be monitored by detecting a signal reflected from theFBG 104 or a signal transmitted through the FBG 104. The Braggwavelength λ_(B), also known as the central wavelength, of the grating104 is determined by:

λ_(B)=2n_(e)Λ_(T)

where n_(e) is the effective refractive index of the grating 104 in thefiber core 106 and Λ_(T) 101 is the grating period. The effectiverefractive index quantifies the velocity of propagating light throughthe core 106 as compared to its velocity in vacuum. A physical propertyassociated with an environment of the grating 104 that can be measuredis referred to herein as a measurand and example measurands includetemperature, strain, pressure, tension, humidity, etc. As the measurandchanges, the grating period 101 also changes which allows the grating104 to indicate this change of the measurand in its local environment.

In practice, light having a broadband spectrum 110 is coupled as aninput to the optical fiber 102 and the grating 104 reflects a portion ofthe broadband input light 110. The center wavelength of the reflectedlight 114 is determined by each fiber Bragg grating and each is uniquewherein λ_(B1), λ_(B2) may be representative of a plurality of reflectedcenter wavelengths. When a measurand affects the grating 104, the resultis that the central wavelength of light reflected by the grating shifts.This spectral shift can be considered as a direct measurement of themeasurand.

Further, as is also depicted in FIG. 1, the portion of the broadbandspectrum 110 not reflected at the grating 104 continues past the grating104 as a transmitted spectrum 112. The transmitted spectrum 112 providesa spectrum of light that may be reflected at one or more subsequent FBGsproviding reflected light signals in wavelength ranges centered arounddifferent central wavelengths, as is discussed further below.

FIG. 2 is a conceptual view of a sensor arrangement of a plurality offiber Bragg gratings that can be used as a strain gauge arrangement inaccordance with the principles of the present invention. Several FBGgratings 204 ₁, 204 ₂, 204 ₃, 204 ₄, and 204 ₅, each with a respectivedifferent center wavelength λ_(B1), λ_(B2), λ_(B3), λ_(B4), and λ_(B5),can be included in the same fiber 202 chain and measured simultaneously.The basic configuration of such an FBG based sensor may include anilluminating source 201, an optical coupler 212, a detection unit 216,and the different FBGs, i.e., 204 ₁-204 ₅. Light 210 from the source 208is reflected from each FBG and the coupler 208 passes this reflectedlight to the detection unit 216. Shifts of the Bragg wavelength aretranslated to changes in the measurand. In particular, a reflectionspectrum 214 of the reflected light includes a respective reflectedsignal from each of the FBGs 204 ₁-204 ₅ with the respective reflectedsignal depending on the respective Bragg wavelengths λ₁ through λ₅ ofthe FBGs, 204 ₁-204 ₅. By knowing the Bragg wavelength of a grating at aknown measurand value, the observed shift of that wavelength can be usedto determine a change in that measurand value.

As shown in FIG. 2, the shift in wavelength of a plurality of FBGs 204₁-204 ₅ can be simultaneously detected and individual shifts of theBragg wavelength will respectively indicate changes in the measurand atthe specific location occupied by that Bragg grating. The number of FBGsthat can be incorporated within a single fiber 202 depends on thewavelength range of operation of each FBG and the total availablewavelength range of the detection unit 216. Because wavelength shiftsdue to strain are typically more pronounced than temperature, FBG strainsensors are often given a ˜5 nm range, while FBG temperature sensors areallotted ˜1 nm. Because typical detectors may provide a measurementrange of about 50 to 100 nm, each fiber array of sensors can usuallyincorporate anywhere from one to more than 80 FBG sensors—as long as thereflected wavelengths do not overlap in the optical spectrum. Thebroadband source 201 can include a light source producing a spectrum 210with wavelengths of between 1500 nm to 1600 nm with a center wavelengthλ₀ of about 1550 nm. One of ordinary skill will recognize that otherbroadband light sources, other operating wavelengths and more or fewerFBGs can be used without departing from the scope of the presentinvention.

Thus, in FIG. 2, the gratings 204 ₁-204 ₅ are chosen so that theirreflected spectrum is within the spectral band of the light spectrum 210provided by the light source 201. The Bragg gratings havenon-overlapping spectral reflection characteristics 214 so that each isidentifiable at the detection unit 216 by its own spectral band. Thereflected Bragg signal from each FBG sensor is monitored in reflectionby the detection system 216. Any shift in the Bragg wavelength willindicate a change in the measurand at the corresponding FBG sensorlocation.

In accordance with aspects of the present invention, the FBG strainsensor may be located on the vertical support gussets of a generator,such as a turbo-generator. At this location, the sensor, along with aplurality of other similar sensors, can be used to detect straininformation beneficial for determining frame foot loading parameters ofthe generator. Thus, according to this aspect of the invention, the FBGsensor is mechanically coupled with a surface of a gusset so that strainon the surface of the gusset can cause resulting strain of the FBGstrain sensor, which can then be detected and measured. FIG. 3 depicts aweldable fiber Bragg grating component 300 in accordance with theprinciples of the present invention.

The weldable FBG component 300 of FIG. 3 may include a pre-stretched orpre-tensioned optical fiber 302, incorporating a FBG 304, which isattached to a weldable plate 303. The weldable plate 303 provides arelatively flat, smooth surface that simplifies bonding of the fiber 302to an appropriate surface. The plate 303 can be anchored to the surfaceby epoxy, cement or other adhesives. However, one beneficial method isto construct the plate 303 out of a metal that can be welded to asurface where strain is to be measured. For example, the plate 303 maybe spot welded to an underlying substrate surface. A weld may provide amore reliable attachment over time as compared to epoxy or cement.

The fiber 302 is typically anchored to opposite ends 303A, 303B of theweldable plate 303 with a respective fiber extension 302A and 302Bextending from each such anchoring point. The fiber extensions 302A,302B can each have a respective connector or optical coupler 320A and320B that allows the FBG component 300 to be easily inserted within as aportion of a multicomponent fiber assembly.

The FBG strain sensor of FIG. 3 is merely an example of one way thatsuch a sensor can be constructed to provide a strain sensor that can beattached to a surface of an object. One of ordinary skill will recognizethat there are other, functionally equivalent methods of constructingFBG strain sensors without departing from the scope of the presentinvention.

FIGS. 4A-C depict a module or modular construction 400 that may beconfigured with one or more fiber Bragg gratings in accordance with theprinciples of the present invention. The module 400 of FIGS. 4A-4C canbe referred to as a frame foot loading (FFL) module. The arrangement ofFIG. 4A includes a fiber chain that may include a first FBG strainsensor 300 ₁ (on the left) and a second FBG strain sensor 300 ₂ (on theright). The strain sensors 300 ₁, 300 ₂ may be constructed similar tothe component 300 described above and, as such, may each include aweldable plate, such as plate 303 described above. The module 400 mayalso include an FBG temperature sensor 404 located in between the twostrain sensors 300 ₁, 300 ₂. The structure of a FBG temperature sensorand a FBG strain sensor is substantially the same with the exceptionthat the weldable plate 303 need not be included for the FBG temperaturesensor 404. In each instance the grating period of each of the FBGschange as a result of a change in a measurand. In particular, for theFBG temperature sensor 404, the effective refractive index, n_(e), ofthe temperature sensor FBG changes as a result of temperature change ofthe fiber 402 thereby shifting the Bragg wavelength.

While there may be couplings and connectors (not shown) between thedifferent FBG sensors 300 ₁, 300 ₂, 404, the effect is that the fiber402 is effectively a continuous optical fiber path between terminaloptical couplers or connectors 422A and 422B for the module 400.

FIG. 4B depicts the FBG sensor arrangement from FIG. 4A attached to aprotective housing or casing for the module 400. The casing can includea bottom plate 430 and a cover plate 432 that provide an enclosure forthe FBG sensor arrangement of FIG. 4A. The terminal connectors 422A,422B can each be coupled with a respective external connector 424A,424B. In this way, the module 400 of FIGS. 4B and 4C can be astand-alone strain sensor module that may include one or more FBGsensors. In particular, as described herein, the module 400 can be astand-alone strain sensor module including a plurality of FBG strainsensors, e.g., two FBG strain sensors, and that may include an FBGtemperature sensor, as well as connectivity points for receiving andtransmitting optical signals.

FIG. 4C depicts one particular feature of the bottom plate 430. It isenvisioned that the bottom plate 430 will be attached in some manner tothe frame gusset and that the inside of the module 400 will beaccessible by removing the cover plate 432. However, attaching the FBGstrain sensors 300 ₁, 300 ₂ to the bottom plate 430 may not accuratelydetect the strain being experienced by the gusset. Thus, respectiveopenings 440, 442 are provided in the bottom plate 430. These openingsallow the respective weldable plates 303 associated with each FBG strainsensor 300 ₁, 300 ₂ to be welded directly to the surface of the gussetwithout any interference from the bottom plate 430. The openings 440,442 are sized appropriately based on the weldable plate 303 size and, asfor spacing, the openings 440, 442 can be separated by a distance ofbetween about 4 inches to about 7 inches. As a result, the module 400,when attached to the surface of the gusset, will provide two differentmeasurements of strain experienced by that gusset surface as well as anindication of the temperature where those FBG strain sensors 300 ₁, 300₂ are located.

The FBG temperature sensor 404 can be securely located within the moduleby the fibers connecting the temperature sensor 404 with the FBG strainsensors 300 ₁ and 300 ₂. Thus, the temperature sensor 404 can avoidbeing affected by the stress or strain of being rigidly mounted to afixed surface. As mentioned, the FBG temperature sensor 404 provides away to measure temperature near the location of the FBG strain sensors300 ₁ and 300 ₂. However, one of ordinary skill will recognize that adifferent type of temperature sensor may be provided as well. Forexample, a wireless network of identifiable, semiconductor-based,temperature sensors would allow temperature to be sensed at differentlocations where the module 400 may be located.

One of ordinary skill will recognize that additional FBG strain sensorscan be included in the module 400 without departing from the scope ofthe present invention. For example, a third FBG strain sensor could belocated between the FBG temperature sensor 404 and the FBG strain sensor300 ₂. In this arrangement, the breadth of the module 400 can beextended to the right to accommodate the additional sensor. In such anarrangement, the fiber 402 can be formed in a serpentine configurationso that all three of the FBG strain sensors are substantially verticallyaligned within the extended module.

FIGS. 5A and 5B are each a perspective view of different areas of aturbo-generator. In FIG. 5A one side of a generator 450 is shown thatshows the gussets near each end of that side of the generator 450;similar gussets are located on the other side of the generator 450 aswell. The gussets of the front right corner of the generator 450 includefirst through fifth gussets 452A-E. These gussets 452A-E are coupledwith a casing 451 of the generator 450 and with the frame foot 458 and,thus, strain on the gussets is indicative of the frame foot loadingexperienced by the generator 450. Typically, strain on the first four ofthe gussets 452A-D may be determined. Also shown in FIG. 5A is one ormore trunnions 454, 456. Using these trunnions and a hydraulic jack, orsimilar means, a corner of the generator 450 can be lifted so that thefoot frame 458 does not sit on any supporting foundation. In this way, abaseline strain experienced by the gusset surfaces 452A-D can bedetermined when there is no load on the frame foot at that corner.

FIG. 5B shows a detailed view of the frame foot 458 that sits on aseating plate 460. The first and second gussets 452A and 452B areillustrated with location, 400 _(L) for respective modules labeled forreference. Shims 462 are inserted between the seating plate 460 and theframe foot 458 to adjust the frame foot loading pattern experienced bythe gussets.

FIG. 6 depicts the first two gussets 452A, 452B having frame footloading modules in accordance with the principles of the presentinvention. One beneficial technique for acquiring frame foot loadinginformation is to place multiple modules 400 on each of the gussets452A-D. For example, on the first gusset 452A, a FBG strain sensormodule 400A₁ is placed on a front side of the gusset 452A and acorresponding module 400A₂ is placed on a back side of the gusset 452A.The terms “front” and “back” are used for convenience to denote oppositesides of the gusset are not intended to limit embodiments of the presentinvention to only specific spatial arrangements.

Similarly, a second pair of modules 400B₁ and 400B₂ are attached to thesecond gusset 452B. Thus, each gusset 452A, 452B has two FBG strainsensor modules so that each gusset 452A, 452B includes 4 FBG strainsensors and 2 FBG temperature sensors. The modules 400A₁, 400A₂, 400B₁,400B₂ may be placed in similar locations on each respective gussetsurface so that misplacement of the modules does not introduceunintended differences in the measurands. In particular, each module canbe placed about 3 to 7 inches from the outside edge of a respectivegusset and about 3 to 7 inches above the frame foot.

An optical fiber 470 can connect the first module 400A₁ to asource/detector, e.g., a source 201/detection unit 216 depicted in FIG.2, and a jumper fiber 472A can connect the first module 400A₁ to thesecond module 400A₂. Another jumper fiber 474A can connect the secondmodule 400A₂ to the first module 400B₁ on the second gusset 452B, and afurther jumper fiber 472B can connect the first module 400B₁ to thesecond module 400B₂. An additional optical fiber 474B can be used toextend the fiber chain to additional modules.

FIG. 7 depicts a corner of a generator frame with a plurality of gussets452A, 452B, 452C, 452D supporting respective frame foot loading modulesin accordance with the principles of the present invention. In FIG. 7,each of four different modules 400A₁, 400B₁, 400C₁, 400D₁ are shownattached to respective gussets. Each of these four different modules hasa corresponding module, which is not visible, on the opposite side oftheir respective gusset, such that there are 8 modules on the generatorcorner of FIG. 7. Also, there are three other corners not shown in FIG.7 that may have a similar arrangement of modules on the respectivegussets of those corners. Thus, the generator of FIG. 7 may have, forexample, 32 modules in total to help determine frame foot loadingparameters.

As mentioned previously, there are instances where modules may beattached to only three gussets at each corner of the generator. One ofordinary skill will recognize that more than 4 gussets may have attachedmodules, as well, without departing from the scope of the presentinvention.

As described above, the modules on one side of a gusset are coupled withthe module on the corresponding opposite side by a corresponding jumperfiber 472A-D. Also, there are jumper fibers 474A-C which represents thecoupling between a module from one gusset to a module on another gusset.These jumper fibers 472A-D and 474A-C are used when strain (andtemperature) measurements of the different modules are being acquired.Before operation of the generator, after frame foot loading has beenadjusted, these jumper fibers 472A-D and 474A-C may be removed. Onebenefit is that the modules 400A₁-D₁, and associated opposite modules,can be left in place so that if the frame foot loading needs to berecalculated at some future time, the only connections needed are to addthe jumper fibers, substantially reducing the complexity and connectiontime over prior measurement systems.

As mentioned earlier, multiple FBG strain sensors can be coupledtogether in series and all analyzed at once. Thus, the signals from the8 modules coupled to the corner gussets shown in FIG. 7 could all beanalyzed contemporaneously. When a different corner is to be analyzed,then connections to those 8 modules can be put into place and thosesignals analyzed. However, in addition to each corner being analyzedseparately, a jumper fiber may be used to couple a “last” module fromone corner to a “first” module of a second corner of the generator asmay be represented by the jumper fiber 474N. In this way, all thecorners (or just some of the corners) of the generator can be linkedtogether so that signals from all the corners could be acquired andanalyzed in a contemporaneous manner.

FIG. 8 is a flowchart of an exemplary method of sensing frame footloading patterns in accordance with the principles of the presentinvention. In a first step, 802, the plurality of FBG strain gaugemodules (i.e., the FFL modules) are attached to appropriate locations ongussets at one or more corners of a generator. The FFL modules are alsolinked together by appropriate jumper fibers to create an optical signalpath that includes multiple FBG strain sensors. The optical signal pathmay also include multiple FBG temperature sensors.

In step 804, the baseline measurements of the FBG strain gauges aredetermined. In particular, the corner of the generator can be lifted sothat any load can be removed from the frame foot (and the supportgussets) at a generator's corner. Each FBG may still experience sometension or strain, even when no load is placed at the frame foot. Forexample, a typical FBG 302, as shown in FIG. 3, is prestretched beforebeing anchored to the weldable plate 303. Also, the welding of the plate303 to a gusset surface may also add additional strain on the FBG 302.Thus, in step 804, the baseline Bragg wavelength of each FBG strainsensor is determined in its installed condition on a gusset under noload.

If a FBG temperature sensor is utilized, as depicted in FIGS. 4A-4C,then a calibration step may be performed so that the Bragg wavelength ofeach temperature sensor at a known temperature can be calculated. Inparticular, the calibration of the FBG temperature sensor can beperformed at the same time the no-load strain measurements aredetermined in step 804.

For determining the baseline strain measurements and the temperaturecalibration measurements, a signal source and detector are coupled withthe string of FFL modules and the reflected signals are used todetermine a baseline, or calibration, Bragg wavelength for each FBG inthe string of FFL modules.

In step 806, the frame feet of the generator are attached, orre-attached, to their respective seating plates. As described above, asignal analyzer having a signal source and detector can be coupled withthe string of FFL modules, in step 808, that provide an optical signalpath. As described above, the signal source transmits a broadband signalthat causes a respective reflected signal from each FBG in the opticalsignal path. Each of the reflected signals may be shifted as a result ofthe strain or temperature differences as compared to the baseline Braggwavelength for a respective FBG sensor.

Thus, in step 810, the reflected signal for a FBG temperature sensor canbe used to determine a compensation factor to be used when evaluating astrain measurement for a particular FBG strain sensor. For example, forone FFL module, there may be two FBG strain sensors and a nearby FBGtemperature sensor. Each strain sensor will experience a respectiveBragg wavelength shift due to the strain experienced by the gussetsurface to which they are attached. The FBG strain sensors will alsoexperience a Bragg wavelength shift due to any ambient temperaturedifferences between their current environment and the environment whenthe baseline strain readings were determined. Thus, the FBG temperaturesensor indicates an amount of Bragg wavelength shift that results fromtemperature differences and this shift can be used to compensate, instep 810, the measurement of the reflected signal from each FBG straingauges so that the Bragg wavelength shift due solely to strain can bedetermined. Thus, in step 812, the load at each gusset, and frame foot,can be calculated accurately under a variety of temperature conditions.Based on the frame foot loading pattern that is calculated, shims andother techniques can be used to adjust, in step 814, the frame footloading to a desired pattern.

As described above, the jumper fibers between adjacent gussets, andbetween corners of the generator, may be removed at their respectivecouplings with the modules before placing the generator in operation.Further, the jumper fibers may be re-attached in a timely and efficientmanner, requiring a relatively few connections, to again perform thesteps described in FIG. 8 at a later time.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

What is claimed is:
 1. A strain measuring device comprising: an opticalsignal path having a first end and a second end; a strain modulecomprising: a first fiber Bragg grating in the optical signal pathbetween the first end and the second end; and a second fiber Bragggrating in the optical signal path between the first fiber Bragg gratingand the second end; and a housing configured to substantially enclosethe strain module; wherein the housing has an outer surface configuredto be mechanically attached to a surface of a support gusset of agenerator.
 2. The device of claim 1, wherein the housing includes acover plate and a bottom plate and an outside surface of the bottomplate corresponds to the outer surface configured to be mechanicallyattached to the surface of the support gusset of the generator, and thebottom plate includes a first and second opening that expose an insideof the housing to the surface of the support gusset.
 3. The device ofclaim 2, wherein: the first fiber Bragg grating comprises a firstoptical fiber anchored to a first weldable plate; the second fibergrating comprises a second optical fiber anchored to a second weldableplate; and wherein the first opening is configured to accommodate thefirst weldable plate to allow the first weldable plate to contact thesurface of the support gusset at a first location and the second openingis configured to accommodate the second weldable plate to allow thesecond weldable plate to contact the surface of the support gusset at asecond location.
 4. The device of claim 1, wherein: the first endincludes a first optical connector configured to be coupled with a firstjumper fiber; and the second end includes a second optical connectorconfigured to be coupled with a second jumper fiber.
 5. The device ofclaim 1, wherein the strain module further comprises a temperaturesensor located proximate to the first and second fiber Bragg gratings.6. The device of claim 5, wherein the temperature sensor comprises athird fiber Bragg grating.
 7. The device of claim 6, wherein the thirdfiber Bragg grating is included in the optical signal path.
 8. Thedevice of claim 7, wherein the third fiber Bragg grating is locatedbetween the first fiber Bragg grating and the second fiber Bragggrating.
 9. The device of claim 7, wherein each respective Braggwavelength of the first, second, and third fiber Bragg gratings aredifferent.
 10. A strain measuring device comprising: a plurality ofmodules, each module comprising: an optical signal path having a firstend and a second end; at least one fiber Bragg grating in the opticalsignal path between the first end and the second end; and a housingconfigured to substantially enclose the at least one fiber Bragggrating; wherein the housing has an outer surface configured to bemechanically attached to a surface of a support gusset of a generator;wherein the plurality of modules are arranged in a sequential chain ofadjacent modules having a beginning module and an ending module toprovide a single optical signal path; a light source coupled with thefirst end of the beginning module and configured to provide an incomingspectrum of light; a detector coupled with the first end of thebeginning module and configured to receive a respective reflected signalfrom each of the plurality of modules corresponding the respective Bragggratings; and a jumper fiber configured to optically couple the secondend of each of the plurality of modules to the first end of itsrespective neighbor in the sequential chain, starting at the beginningmodule and stopping at the ending module.
 11. The device of claim 10,wherein the plurality of modules are logically grouped together as pairsof modules such that for a particular pair of modules a first module ofthe pair is configured to be attached to a first side of a respectivegusset and the second module of the pair is configured to be attached tothe second side of the respective gusset.
 12. The device of claim 11,wherein the at least one fiber Bragg grating comprises a first fiberBragg grating, and and each of the plurality of modules furthercomprise: a second fiber Bragg grating in the optical signal pathbetween the first fiber Bragg grating and the second end; and atemperature sensor located proximate to the first and second fiber Bragggratings.
 13. The device of claim 12, wherein for each of the pluralityof modules: the housing includes a cover plate and a bottom plate and anoutside surface of the bottom plate corresponds to the outer surfaceconfigured to be mechanically attached to the surface of the supportgusset of the generator, and the bottom plate includes a first andsecond opening that expose an inside of the housing to the surface ofthe support gusset, wherein: the first fiber Bragg grating comprises afirst optical fiber anchored to a first weldable plate; the second fiberBragg grating comprises a second optical fiber anchored to a secondweldable plate; and wherein the first opening is configured toaccommodate the first weldable plate to allow the first weldable plateto contact the surface of the support gusset at a first location and thesecond opening is configured to accommodate the second weldable plate toallow the second weldable plate to contact the surface of the supportgusset at a second location.
 14. The device of claim 12, wherein foreach of the plurality of modules: the temperature sensor comprises athird fiber Bragg grating included in the optical signal path andlocated between the first fiber Bragg grating and the second fiber Bragggrating.
 15. The device of claim 14, wherein each respective Braggwavelength of the first, second, and third fiber Bragg gratings aredifferent.
 16. A method of determining frame foot loading of a generatorincluding a generator casing supported on a plurality of frame feet andincluding gussets extending between the generator casing and the framefeet, the method including: attaching at least one frame foot loadingmodule to each of a plurality of the gussets, the plurality of modulesbeing arranged in a sequential chain of adjacent modules having abeginning module and an ending module to provide a single optical signalpath; each module comprising: an optical signal path having a first endand a second end; at least one fiber Bragg grating in the optical signalpath between the first end and the second end; and a housing configuredto substantially enclose the at least one fiber Bragg grating; whereinthe housing has an outer surface configured to be mechanically attachedto a surface of a support gusset of a generator; coupling a light sourcewith the first end of the beginning module, the light source configuredto provide an incoming spectrum of light; coupling a detector with thefirst end of the beginning module, the detector configured to receive arespective reflected signal from each of the plurality of modulescorresponding to the respective at least one fiber Bragg grating of eachof the plurality of modules; and attaching a respective jumper fiber tooptically couple the second end of each of the plurality of modules tothe first end of its respective neighbor in the sequential chain,starting at the beginning module and stopping at the ending module. 17.The method of claim 16, including determining a frame foot loading forat least one frame foot and removing the jumper fibers prior tooperation of the generator.
 18. The method of claim 17, includingre-attaching the respective jumper fibers between the modules to performa further determination of a frame foot loading following operation ofthe generator.
 19. The method of claim 16, wherein a plurality ofgussets on at least two of the frame feet are provided with the modules,such that all of the modules on the at least two frame feet areconnected in the single optical signal path.
 20. The method of claim 16,wherein said step of attaching said at least one frame foot loadingmodule to each of a plurality of the gussets includes: coupling a firstframe foot loading module to a first side of a first gusset of thegenerator; coupling a second frame foot loading module to a second sideof the first gusset of the turbo generator; optically coupling the firstframe foot loading module to the second frame foot loading module;determining a first temperature compensation factor associated with thefirst frame foot loading module; determining a second temperaturecompensation factor associated with the second frame foot loadingmodule; detecting a first strain measurement corresponding to the firstframe foot loading module; detecting a second strain measurementcorresponding to the second frame foot loading module; compensating thefirst strain measurement based on the first temperature compensationfactor; compensating the second strain measurement based on the secondtemperature compensation factor; and calculating a frame foot loadingpattern based on the compensated first and second strain measurements,wherein the first and second frame foot loading modules each,respectively, comprise: one of the optical signal paths having a firstend and a second end; a first fiber Bragg grating in the optical signalpath between the first end and the second end; a second fiber Bragggrating in the optical signal path between the first fiber Bragg gratingand the second end; a third fiber Bragg grating comprising a temperaturesensor located in the optical signal path proximate to the first andsecond fiber Bragg gratings; and a housing configured to substantiallyenclose the first, second and third fiber Bragg gratings.